There is an aspect to this case that has been missed. The photographer is German. I see no reasons why he would or should enforce his rights in the IPEC. If the infringement was visible in Germany then the copyright holder can action the case in the German courts. ( See Pez Hejduk. v. EnergieAgentur.NRW GmbH )
Be aware Germany has very strict copyright laws which is why copyright infringement is relatively rare over there. One of the reasons is that under German Law you are required to sign a contract, called an Unterlassungserklärung , promising to remove all infringing copies and never infringe again. these contracts have sever penalties for breach.
The UK is unusual in having a Small Claims Track where legal costs are not recoverable. In Germany as with most jurisdictions outside the UK, legal from a professional lawyer are fully recoverable in court and they start with paying for the very first letter.
Going back to the original case, you mention the image was attached to a Creative commons licence, are you sure this was authorised by the copyright holder? The internet is full of photographs purportedly being offered as Creative Commons where the copyright holder has granted no such licence. In those cases the end user is still liable.
If the photographer has granted a Creative Commons licence then his case is weak. I wouldn't normally suggest figures for negotiation, but if this is correct and the photographer has allowed free unpaid use of his work on the basis of a credit, then I would offer 100 Euros as settlement. Given the photographer is in Germany, i would suggest you make an offer asap as you don't want them to start legal proceedings over there which would cost vastly more in legal costs than the damages being argued about.
On the other had if the photographer is a genuine professional photographer and did not offer his works under a CC licence then he is fully entitled to be paid for his work. Many if not most German Professional Photographers work to an industry standard price list called the Mittelstandsgemeinschaft Foto-Marketing (mfm). You can buy a copy of this price list from the BVPA ( A Trade Association for picture sellers )
If you have received a "Mahnung" with a demand for both payment and signing an "Unterlassungserklärung" then you should engage a specialist German IP lawyer immediately, its a serious matter and not something to solve through internet forum advice. Unlike the UK, German deadlines are serious and there are consequences for not meeting them. A decision by the German courts is easily enforceable in the UK and if it goes all the way to baliffs then even a low value case will be eye wateringly expensive by the time there is a knock on the door. The good news is Lawyers appear to be cheaper in Germany and seem much more professional in seeking an out of court settlement rather than encouraging needless litigation like some do in the UK. Also all or nearly all will speak English.
If you are worried about infringement or your work has been copied and you want to take action.